This invention relates to reflection type probes for use with eddy current testing instruments. Such instruments, as is well known, are used for a variety of purposes involving non-destructive testing, such as crack detection and conductivity and thickness measurements, in bodies of electrically conducting material.
Reflection type probes comprise basically a former on which are mounted a driver coil system energised from an alternating current supply and a pick-up coil system which generates an e.m.f. as a result of its coupling with the driver coil system through the alternating magnetic field generated by the driver coil system. The former may be of ferromagnetic material or the coil systems may be air cored. When the probe is brought adjacent an electrically conducting body, eddy currents generated in the body modify the electromagnetic field and produce a change in amplitude and phase of the generated e.m.f. Both the amplitude of the generated e.m.f. and the phase relationship with the driving signal varies in dependence upon the proximity of the probe to the electrically conducting body, i.e. they are sensitive to so-called lift-off. However the output from the coil systems is connected to appropriate electronic circuits which operate so as to keep the phase relationship sensibly constant. For this reason the instruments wih which the probes are used are usually designed tomeasure phase-shift when performing their testing function.
Probe designs are known in which the pick-up coil system is in two opposingly wound coil sections so that in air the e.m.fs generated across the two coil sections cancel each other out. In use of such a probe one of the coil sections becomes disposed nearer to the body to be tested than the other and serves as the detector coil section sensitive to the eddy currents generated in the body. The other coil section is not significantly influenced by the eddy currents but will be subjected, in theory at least, to the same extraneous influences as the detector coil section. The e.m.fs generated in the two coil sections as a result of extraneous influences will therefore cancel each other out so that in theory the effect of extraneous influences on the pick-up coil system will be nullified. For convenience the other coil section of the pick-up coil system will be referred to as a "balancing coil section".